1. Field of the Invention
The present invention relates to a printing apparatus that has a head, which enables at least two types of dots having different ink densities and ink weights to be recorded, and prints a multi-tone image with inks spouted from the head. The present invention also pertains to a printing method and a recording medium for realizing the above functions.
2. Description of the Related Art
Color ink jet printers that spout inks of plural colors from the respective ink spout heads are widely used as the output apparatus of the computer. The color ink jet printer prints multi-color multi-tone images processed by the computer. One proposed technique applicable for such a printing apparatus and the corresponding printing method utilizes a higher-density ink and a lower-density ink, with a view to further improving the printing quality in a highlighted area or in an area of low image density. This technique provides a higher-density ink and a lower-density ink for an identical color and controls the spout of these inks, so as to realize printing of excellent tone expression. Another proposed printing apparatus creates dots of different ink weights by the respective inks.
Another proposed technique for expressing the multi-tone creates two different types of dots having different ink densities and different ink weights, so as to vary the density per unit area in multiple stages (for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 59-201864). This technique causes each pixel to consist of four dots and varies the frequency of appearance of the higher-density dots and the lower-density dots in the pixel, in order to print an image of multiple densities.
In a printer that creates dots to record an image, the mechanical manufacturing error of the head that spouts ink and creates dots may cause banding, that is, unevenness of formation of dots, in the head-reciprocating direction or in the main scanning direction. The banding at any tone level deteriorates the picture quality. The effect is especially pronounced in the half tone area that has a relatively small number of dots. In the printer that records an image with dots, it is important to prevent such a banding for the improvement in picture quality. A variety of techniques have been proposed to cancel the banding, separately from realizing the multi-toning, which also leads to an improvement in picture quality. One example is the overlap method that forms each raster by a plurality of main scans of the head.
In the printing apparatus that can create two different types of dots having different ink densities and ink weights, however, no special attention is given to cancel the banding and improve the picture quality. This conventional printing apparatus creates the two different types of dots corresponding to the input tones of the respective pixels according to a predetermined pattern. It is not determined which one of the two types of dots is to be created for each dot unit, based on a variety of conditions for recording an image with dots.
The conventional printing apparatus makes the density per unit area for the dots of a larger ink weight generated by the light ink lower than that for the dots of a smaller ink weight generated by the dark ink, in order to realize multi-toning and improve the picture quality. The density of each pixel is expressed by a combination of these different types of dots. In this arrangement, the dots to be used depend upon only the tone values of the image data, and there is a relatively restricted degree of freedom for the section of the dots. This may cause an inadequate printing result. In general, dots of a smaller ink weight are liable to cause banding. The banding often arises at the tone level where a large number of small dots are created. The larger number of small dots may also result in exceeding the allowable quantity of ink per unit area of the paper, that is, the ink duty of the paper.